The field of the invention relates to the manufacture of eyeglass lenses and preforms and molds relating thereto.
This improved invention relates to preform lens blanks, and to a method for making a lens, more particularly to a system embodying the cold flow compression molding of a lens between a pair of mold/cavities using a preformed lens blank of thermoplastic material, and of predetermined configuration. The system is particularly applicable for use in house by the optician, optometrist and/or ophthalmologist. Alternatively, the system can be used in a specialized lab. The method and apparatus do not require the use of skilled technicians, and reduces the cost of prescription lenses since they are created substantially defect free, eliminating the need for any middle men, such as a laboratory or lens factory. Moreover, the lens can be fabricated quickly in minutes, so that the patient or customer can be fitted with a pair of prescription glasses in virtually no time, and certainly less time than it takes for an eye examination.
According to the invention a method for forming a thermoplastic preform into a lens by compression molding between a pair of molds is provided. In one aspect of the invention a set of novel preforms made from a thermoplastic material is provided. The preforms have different radii of curvature. Preferably each different curvature preform comes in different weights desirably two or three different weights.
A series of mating concave and convex mold halves are provided for use with the preforms. According to the invention a limited number of concave mold halves are provided. A wide range of single vision and multifocal lenses can be supplied from three (3) to six (6) base curve concave mold halves having different radii of curvatures. According to the invention a first concave mold half with a radius of curvature of less than two (2) diopters is provided. Each successive concave mold half has a radius of curvature of from one (1) to three (3) diopters desirably about two (2) diopters greater than the previous mold half in the series.
Thermoplastic preforms are associated with the different concave mold halves. Each preform has a convex side for placement in a concave mold half and a flat side or substantially flat side or a concave curve side for engagement with a convex mold half
The convex side of the preform has a radius of curvature in diopters of 1 to 5 diopters greater than the radius of curvature of the concave mold half. Desirably the radius of curvature of the preform is 2 to 4 diopters greater than the radius of curvature of the concave mold desirably about 3 diopters or above and preferably 3 diopters greater than that of the concave mold. A convex mold half is provided for mating with the concave mold to complete the finished lens having a desired magnification strength.
According to the invention one of the limited number of concave mold halves is selected depending on the desired prescription of the lens. The appropriate preform is also selected depending on the mold and prescription involved. The preform and mold are heated to a temperature above the softening point of the preform. Desirably the preform is placed in the concave mold half and preheated to a temperature above the softening temperature of the lens. The preform and the mold are placed in a press. The molds and the preform can be preheated before or after being placed in the press. After the molds and the preform have been heated above the softening point of the thermoplastic material of the preform, pressure is applied by the press. A central thickness of the lens is preselected, depending on the prescription. The press is operated to compress the lens to the preselected central thickness. Multiple stages or a single stage can be used. Desirably the press is operated in two stages. Desirable, in the first stage, about 10% to 50% of the desired displacement of the lens is achieved. In the second stage, additional pressure is applied until the preselected center thickness of the lens is achieved. After the lens has been suitably compression molded, the lens is cooled, edged, trimmed and coated as desired for use.
In another aspect of the invention, novel preforms are provided. According to the invention, a preform is provided having an indent on its flat or concave side. The indent centers the convex mold half during the compression process and prevents excess flash. The indent prevents incomplete lenses from forming and the lens from decentering. In another aspect of the invention, preforms are provided with locator arms attached to the periphery of the preform for mating engagement with the mold base used to hold the bottom mold. The locator arms align the preform in a preselected orientation to a concave mold located in the mold base. The locator arms prevent shifting of the lens after it has been oriented. Such shifting or decentering can result in unwanted introduction of prisms into the lenses and may result in erroneous stigmatism corrections or misplacement of the position of a bifocal.
The preferred embodiment of the present invention is illustrated in the drawings and examples. However, it should be expressly, understood that the present invention should not be limited solely to the illustrative embodiment.